This invention relates to a polymer excellent in transparency, low moisture-absorption properties, and heat resistance.
Heretofore, poly(methyl methacrylate) resins, polycarbonate resins, polystyrene resins, methyl methacrylate-styrene copolymer resins have been known as transparent resins. These resins are used as molding materials or sheet materials for producing electrical and mechanical parts, automotive parts, light fitment parts, and general merchandise goods applying their transparency, thermoplastic properties, and the like. Further, these resins are going to be used newly as materials for optical elements such as various optical lenses, optical fibers, optical connectors, optical disks, optical circuits, and the like. But with enlargement of application fields, demands for properties of these transparent resins are varied and enhanced, for example, there are newly or highly demanded low moisture absorption properties, heat resistance, molding transfer properties, low birefringence of molded products, and the like in addition to the transparency originally required.
But new materials satisfying all such demands have not been developed, and this is an unsolved problem. For example, poly(methyl methacrylate) resins are excellent in optical properties such as transparency and low birefringence but has a fatal defect in moisture absorption properties. Polystyrene resins are excellent in low moisture absorption properties but disadvantageously large in birefringence and poor in heat resistance. Further, polycarbonate resins are excellent in heat resistance but disadvantageously very large in birefringence like polystyrene resins.
In order to improve the moisture absorption properties and heat resistance of poly(methyl methacrylate) resins which have high posibility as materials for optical elements among these resins, there have been proposed processes for copolymerizing cyclohexyl methacrylate or benzyl methacrylate with methyl methacrylate (e.g. Japanese Patent Appln, Kokai (Laid-Open) Nos. 5318/83, 5354/83, 11515/83, and 13652/83). According to these processes, the moisture absorption properties can be improved considerably but are still insufficient as materials for optical elements and there arises another problem of lowering the heat resistance remarkably.